A. Field
The invention relates to an apparatus for processing sheet material, in particular a bank note processing machine, wherein the sheet material is held between a conveyor belt and further transport devices disposed opposite the conveyor belt and guided through between at least two components of a checking device disposed opposite each other.
B. Related Technology
The conveyor belt serves for guiding and for pressing the sheet material against the transport devices disposed opposite and against guiding paths or checking devices disposed in between. The transport devices usually are transport rolls or transport rollers or a second conveyor belt, which with the help of deflection rollers is either led back directly before and behind the checking device or is led around the checking device.
As to maintain a reliable transport function even with fluctuating sheet thickness, in DE 198 40 420 A1 is proposed that the run of the first conveyer belt facing the material is loaded elastically in the direction of the sheet material by pressure elements. In DE 29 23 148 A1 it is furthermore proposed to guide the second conveyor belt on deflection rollers in grooves, so that the speed of the sheet material is determined by the first conveyor belt located external in deflection direction. The internal, second conveyor belt merely serves for supporting the sheet material between adjacent deflection rollers, whereas such support instead can also be obtained by a transparent glass plate replacing the second conveyor belt, when the sheet material is to be accessible over its entire width for checking purposes.
The first conveyor belt assuming guiding and pressing function in the area of the checking device, however, impedes the carrying out of a transmission measuring at the sheet material. A soiling measurement is also impeded, since for such a measurement the sheet material usually has to be illuminated from two sides. Therefore, the first conveyor belt usually is replaced by a plurality of narrow belts extending in parallel side-by-side in transport direction and spaced apart from each other. But, however, also with this a full areal measurement of the sheet material over its entire width is not possible. The positions of the belts are rather perceived as dark longitudinal stripes when evaluating e.g. transmitted light.
As to ensure free accessibility for checking purposes, therefore, the transport path must be interrupted and the sheet material measured in free flight by the checking device. Such a measurement during free flight, however, is problematic in the case of low transport speeds and in particular in the case of limp sheet material. Here gravity can incalculably influence the sheet material and lead to the blocking of the machine. DE-A 26 55 580 describes such a free-flight path, the bank notes being conveyed clamped between a plurality of belt pairs. Before and behind the checking device the belts are led away from the transport path via deflection rollers, so that the sheet material passes the checking device in free flight. Here the belt pairs located opposite each other are disposed to each other so as to fill gaps and slightly mesh with each other. Thereby onto the transported bank notes is impressed a reversible wave profile perpendicular to the transport direction which leads to an increase of the stiffness of the sheet material in the transport direction. In this way the free-flight path can be reliably covered. The profiling can be effected by the transport belts, but it can also be obtained by elastic rings disposed on the deflection rollers, the circumference of which projecting in radial direction of the deflection rollers over the transport belts. But it becomes immediately obvious, that the checking of a sheet material deformed in a wave-shaped fashion cannot be carried out in a very reliable and regular fashion.
Therefore, it is the problem of the present invention to propose an apparatus for processing sheet material, wherein the sheet material is reliably guided past a checking device in such a way that it is accessible from both sides over its entire width and reliably checkable.